Landfill Gas

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Every year the world produces a huge amount of organic waste mostly of domestic, commercial, industrial and agricultural origin, which represents a highly unstable and uncontrolled mixture of paper, cardboard, food waste, plastic, rubber, glass, debris, metals and others. 400-450 million tons of municipal solid waste (MSW) forms only in cities, one resident has an average of 250-700 kg/year. The amount of MSW increases annually by 3-6%, which is significantly greater than the rate of growth of the world’s population.

Mechanical sorting of MSW is technically difficult and has not found wide application yet. Direct processing or combustion of huge amounts of waste is technically quite problematic, ecologically dangerous and economically inefficient. The dominant method of waste processing has been and remains the disposal of MSW at landfills and special sites. These sites located near cities and accept municipal waste. In such conditions waste is exposed to intensive biodegradation, which is accompanied by release of landfill gas.

Methane (CH4) and carbon dioxide (CO2) are macro components of landfill gas, their ratio may vary from 40-70% to 30-60% accordingly. Nitrogen (N2), oxygen (O2), hydrogen (H2) and various organic compounds are present as attendant components. The composition of landfill gas conditions a number of its specific properties. The composition of landfill gas causes a number of its specific properties. First of all, the landfill gas is flammable as its caloric value is about 5500 Kcal per m3. It is toxic in certain concentrations. Specific toxicity indexes are determined by the presence of a number of trace contaminants, such as hydrogen sulfide (H2S). Usually landfill gas has a strongsmell.

The average gas generation in the landfill body run out within 10-50 years, at that the specific gas yield is 120-200 m3 per 1 ton of MSW. Typically, the most intense process of bioconversion of waste takes place within the first 5 years, during which about 50% of the total gas reserve is produced. At the same time the landfill gas is really dangerous because of the risk of fire or explosion as well as the negative impact on human health.

The most effective way to reduce emissions of methane from landfills is its collection and disposal. In the early 80s in many countries people began to extract landfill gas in order to prevent environmental problems, fires and explosions. Later production of electricity out of landfill gas became very common. Implementation of projects for energy recovery of landfill gas helps to reduce greenhouse gases emission and air pollutants which positively affects the quality of air and reduces the potential risk to human health. Such projects also reduce energy dependence, create jobs and help to develop local economies. Internationally there are considerable possibilities for expansion of the use of landfill gas energy.

Preparing modern MSW site includes bottom packing and waterproofing, drainage system for leachate tapping, laying of pipes for collection of landfill gas.

MSW layers form on the site during transport delivery. Formed during the bioconversion of organic components of MSW landfill gas is supplied through the vertical holes to the manifold. Then gas forcibly moves to the gas conditioning station for separation from gas condensate. Prepared gas is sent to gasholder or directly moves into cogeneration station module. Electrical and thermal energy produced by generators is then supplied to the grid.

Quantitative indexes:

The gas produced from 1 mln. tons of MSW provides the engine run with electrical capacity of 800-1000 kW during 15-20 years. It is possible to get 150-250 m3 of landfill gaswith 60-80% methane content from 1 ton of MSW.

In world practice there are following methods of utilization of landfill gas:

flaring, that provides removal of odor and lowering of fire hazard on site landfill, at the same time the energy potential of landfill gas isn’t used for economic purposes;

direct combustion of landfill gas for thermal energy production;

the use of landfill gas as a fuel for gas engines to produce electricity and heat;

the use of landfill gas as a fuel for gas turbines to produce electricity and heat;

upgrading of the landfill gas (enrichment) to 94 -95% of methane content, followed by its use in gas networks for general use.

Expediency of application one or another method of utilization of landfill gas depends on specific conditions of economic activity at MSW site and is defined by the presence of solvent consumer of energy derived from landfill gas. In most developed countries this activity is supported by government by means of special laws. Economic indexes of projects of landfill gas extraction could be quite profitable, especially in case there is industrial consumer of landfill gas near the site.